1. Field of the Invention
The present invention relates generally to a numerical control unit for machine tools, and more particularly to a numerical control unit having a load monitoring function for monitoring a load on a tool drive source during a machining operation.
2. Description of the Related Art
In the field of machine tools, various methods have been proposed for detecting the cutting resistance acting on a tool during a machining operation, as a load applied to a spindle for turning the tool or a feed shaft for feeding the tool, and for controlling the operation of the spindle or feed shaft, depending on the magnitude of the load, so as to prevent tool breakage. In general, the cutting edge of a tool will become worn by successive use, and will finally become broken. In the case where the edge of a tool becomes worn, the load applied to a corresponding tool spindle or tool feed shaft will increase during cutting by this tool on a workpiece. Therefore, it is possible to prevent an unexpected tool breakage by monitoring the load on the drive source of the tool spindle or tool feed shaft, judging that tool breakage is liable to occur when the load rises above a predetermined reference value, and thus promptly taking measures such as halting the drive source.
For example, Japanese Unexamined Patent Publication (Kokai) No. 7-51991 (JP7-51991A) discloses a tool breakage preventing system in a numerically controlled machine tool, wherein the disturbance torque acting on at least one of a spindle for turning a tool and a feed shaft for feeding a tool is estimated, the estimated disturbance torque (or alternatively, a composite disturbance torque obtained by combining two estimated external torques of the spindle and feed shaft) is compared with a preset reference torque, and thus, based on the result of comparison, the feed speed of the tool is reduced, the tool feed is halted, or a tool exchange command is output. In one embodiment of this system, a two-stage measurement can be taken after the judgment on the comparison result, in that the estimated (or composite) disturbance torque is compared with two large and small preset reference torques (both being at an abnormal-load detectable level), and that, in the case where the estimated disturbance torque is in between these two reference torques, the feed speed of the tool feed shaft is reduced and, in the case where the estimated disturbance torque exceeds the larger reference torque, the tool feed is halted or the tool exchange command is output.
As disclosed in JP7-51991A, in the conventional method of monitoring a load torque to prevent tool breakage, the reference value, preset as a comparative value relative to a monitored load torque, is at an abnormal-load detectable level (e.g., the value corresponding to the load torque resulted from a tool, the extent of wear of which has reached a level just before tool breakage). Also, the above-described conventional method is constituted so as to monitor the load torque applied on the spindle or feed shaft for preventing tool breakage, and not to monitor the state of cutting of a workpiece by a tool. In this connection, the cutting resistance acting on a tool during cutting will sometimes temporarily increase at the instant when some type of abnormality occurs in the tool or machine tool. In such a case, cut-surface accuracy is liable to deteriorate locally in the area where the cutting resistance increases. However, if the increase in load on the spindle or feed shaft due to such a cutting abnormality does not reach the reference value preset for preventing tool breakage, it is difficult for the conventional method to detect such a cutting abnormality.
In general, tool wear proceeds gradually along with tool usage, and the cutting resistance acting on the tool increases along with progression in wear. In the state wherein a tool is new (i.e., the extent of wear is low), the cutting resistance is small, and thus the load acting on the spindle or feed shaft is also small. In this low wear state, the temporarily increased load acting on the spindle or feed shaft when a cutting abnormality occurs will seldom reach the reference value preset for preventing tool breakage, and therefore it is difficult for the conventional method to detect a cutting abnormality which locally reduces the machining accuracy. In other words, successive use of the tool results in the progression of wear and in a gradual increase in the cutting resistance acting on the tool, so that if a judgment criteria is fixed, it is difficult to judge between a normal cutting state and an abnormal cutting state by way of the magnitude of the load acting on the spindle or feed shaft.